1. Field of the Invention
The present invention relates to inductive load driving electronic circuits using semiconductor power devices for driving the load and, more in particular, to circuits of this kind monolithically integrated.
2. Discussion of the Prior Art
The fact that the driving of strongly reactive loads has the problem of transients is well known. When current is suddenly interrupted on a grounded inductive load, a negative overvoltage develops across the load due to the discharge of energy stored by the inductance during a charging phase (at the beginning of current flow through the inductive load). These negative voltage peaks, if unchecked in intensity, may produce breakdown of junctions and, in case of integrated circuits, may cause the triggering of parasitic transistors and of related "latch-up" problems. The remedy is that of providing means for discharging the energy stored in the inductance at a negative voltage lower than or equal to one VBE.
The prior art contemplates substantially two types of solutions:
a first solution contemplates the connection of a recirculation diode across the inductive load in order to provide a recirculation path to the discharge current of the stored energy;
another type of solution is recirculation of the discharge current of the energy stored in the inductive load through the driving power transistor itself, by connecting a control zener diode between the base and the collector of the power transistor.
The first of these known solutions has the drawback of requiring at least an additional power device, namely the recirculation diode, capable of sustaining the passage of a discharge current peak.
The second of these known solutions, though utilizing advantageously the same driver transistor for recirculating the discharge current, requires that the controlling zener diode be designed "ad hoc" in function of the supply voltage of the circuit in order to prevent interference during the charging of the inductance.
Lately active circuits have also been proposed for "switching-ON again" the power transistor driving the inductive load when the negative overvoltage across the latter rises over a certain value preset by a constant voltage generator. Also such a system is not free of drawbacks such as dependency of the triggering threshold of the control active circuit from parameters such as temperature and/or the supply voltage of the circuit.